The Questions of Developmental Biology

10. Don't regress your tail until you've formed your hindlimbs.
11. There are several types of evidence. Correlation between phenomenon A and phenomenon B
does not imply that A causes B or that B causes A. Loss-of-function data (if A is experimentally
removed, B does not occur) suggests that A causes B, but other explanations are possible. Gainof-function
data (if A happens where or when it does not usually occur, then B also happens in
this new time or place) is most convincing.
12. Protostomes and deuterostomes represent two different sets of variations on development.
Protostomes form the mouth first, while deuterostomes form the anus first.
3. Principles of experimental embryology
3. Principles of experimental embryology
Descriptive embryology and evolutionary embryology both had their roots in anatomy.
At the end of the nineteenth century, however, the new biological science of physiology made
inroads into embryological research. The questions of "what?" became questions of "how?" A
new generation of embryologists felt that embryology should not merely be a guide to the study
of anatomy and evolution, but should answer the question, "How does an egg become an adult?"
Embryologists were to study the mechanisms of organ formation (morphogenesis) and
differentiation. This new program was called Entwicklungsmechanik, often translated as "causal
embryology," "physiological embryology," or "developmental mechanics." Its goals were to find
the molecules and processes that caused the visible changes in embryos. Experimentation was to
supplement observation in the study of embryos, and embryologists were expected to discover the
properties of the embryo by seeing how the embryonic cells responded to perturbations and
disruptions. Wilhelm Roux (1894), one of the founders of this branch of embryology, saw it as a
grand undertaking:
We must not hide from ourselves the fact that the causal investigation of organisms is one of the
most difficult, if not the most difficult, problem which the human intellect has attempted to solve .
. . since every new cause ascertained only gives rise to fresh questions regarding the cause of this
cause.
In this chapter, we will discuss three of the major research programs in experimental
embryology. The first concerns how forces outside the embryo influence its development. The
second concerns how forces within the embryo cause the differentiation of its cells. The third
looks at how the cells order themselves into tissues and organs.
Environmental Developmental Biology
The developing embryo is not isolated from its environment. In numerous instances,
environmental cues are a fundamental part of the organism's life cycle. Moreover, removing or
altering these environmental parameters can alter development.